A radar system usually comprises a transmitter, an antenna and a number of waveguide transmission lines connecting the transmitter to the antenna. Pulses generated by the transmitter propagate through the transmission lines to the antenna for transmission. In the case of a surveillance aircraft such as the Navy E-2C with a TRAC-A rotodome antenna, a rotary coupling is interposed between the transmission lines and the antenna, which rotates. The rotary coupling is able to adapt the stationary transmission lines to the rotating antenna. In such a system, when there ar problems in the transmission path, for example, anywhere between the beginning of the transmission lines and before the antenna, when a pulse is transmitted from the transmitter, the fault in the transmission path would cause a high Voltage Standing Wave Ratio (VSWR) wave to be reflected back to the transmitter. Sensors located near the transmitter would sense the high reflected power and, to save the transmitter from damage caused by the reflected power, cause the radar system to shut down. Some of the most common problems which may occur in the transmission lines include shorts, corrosion and loose couplings.
Present aircraft built-in test equipment is incapable of unambiguously isolating a fault in the transmission path which, for example, may include the transmission lines, the rotary coupling and the antenna, during a high VSWR condition that produces a radar inhibit. The conventional method of isolating the fault in the transmission path includes removing and replacing each of the suspect components, followed by a functional test of the radar system during an actual test flight. Needless to say, this method of fault isolation is extremely time consuming and produces excessive aircraft down time. Moreover, since most of the components are high cost items, by taking these components apart and reassembling them causes possible maintenance induced failures such as the mismating of the components.